An ideal semiconductor laser that would emit high power into a single spectral mode with diffraction-limited output profile is of great interest in a number of applications, including spectroscopy or wavelength multiplexing in telecommunications. However, known laser configurations have been unable to provide the desired diffraction-limited output profile without sacrificing power output, and vice versa. For example, in one approach, a distributed-feedback (DFB) configuration using a one-dimensional (1D) diffraction grating parallel to the laser facets provides high spectral purity when the waveguide is sufficiently narrow, e.g. on the order of 2–5 μm for lasers emitting at 0.8–1.55 μm wavelengths, thereby suppressing higher-order lateral modes with respect to the fundamental mode. Scaling up the stripe width provides increased power but has the drawback of producing a loss of phase coherence across the DFB laser stripe, primarily due to the self-modulation of the refractive index in the active region by non-uniformly distributed carriers. The laser output spectrum is undesirably broadened and limited by the width of the gain spectrum, producing a rapidly diverging, often double-lobed, far-field pattern.
The use of two-dimensional (2D) gratings has been studied employing coupled-mode theory, e.g. “Proposed cross grating single-mode DFB laser”, M. Toda, IEEE J. Quantum Electron. (1992) and “Two-dimensional rectangular lattice distributed feedback lasers: a coupled-mode analysis of TE guided modes”, H. Han and J. J. Coleman, IEEE J. Quantum Electron. (1995). However, these approaches were directed to superimposing 1D gratings in lieu of using actual 2D gratings which allows only two diffraction processes. Also, realistic device geometries and the critical role played by the linewidth enhancement factor (LEF) were not considered.
Another approach, described in U.S. Pat. No. 3,970,959 to Wang et al., is directed to utilizing a DFB laser with a 2D grating to produce periodic perturbations of an acoustic wave by the photo-elastic effect. The approach, however, merely involved varying the refractive index without disclosing device parameters or a 2D lattice structure. Other publications, such as “Coherent two-dimensional lasing action in surface-emitting laser with triangular-lattice photonic crystal structure”, M. Imada et al., Appl. Phys. Lett. (1999), while disclosing experimental demonstrations of the 2D distributed feedback (DFB) lasers, are limited to surface-emitting schemes.
Yet another approach disclosed in OSA Topical Meeting on Advanced Semiconductor Lasers and Applications, Paper AWA6, Kalluri et al. (1999) is directed to an edge-emitting device with a 2D photonic crystal grating. However, the selected geometry, with the facets being tilted relative to the grating to achieve emission normal to the facet, produces an output consisting of two beams emerging at large angles to normal, not a single near-diffraction-limited beam normal to the facet.
The α-DFB laser, disclosed in “Theory of grating-confined broad-area lasers”, J. Lang, K. Dzurko, A. A. Hardy, S. DeMars, A. Schoenfelder, and D. F. Welch, IEEE J. Quantum Electron. (1998), is directed to a configuration in which both the diffraction grating and the gain stripe are tilted with respect to the laser facets. However, both the cavity facets and the grating are necessary to produce optical feedback and laser oscillation is established when the beam approaches the facet nearly at normal incidence, since only then is the optical wave strongly reflected back into the cavity along the same path for feedback. It is primarily this reflection of only a narrow angular cone at the facets that leads to diffraction-limited output beams for much wider pump stripes than normally attainable in the Fabry-Perot geometry. A problem with this is that none are robust enough to maintain a single mode under all conditions of interest.
In the approach disclosed in “Single-mode spectral output observed under cw pumping in shorter-wavelength α-DFB lasers”, A. M. Sarangan et al, IEEE J. Quantum Electron. (1995) and “Far-field characteristics of mid-infrared angled-grating distributed-feedback lasers”, I. Vurgaftman et al., J. Appl. Phys., (2000), the near-IR laser line broadened in pulsed mode while mid-IR devices exhibited little spectral narrowing. This indicates that the spectral selectivity of α-DFB lasers is in general considerably lower than in other DFB lasers. A small deviation in the angle of orientation translates into a large (on the scale of the gain bandwidth) shift in the emission wavelength. Angled-grating (α-DFB) device configurations have been disclosed, for example, in U.S. Pat. No. 5,337,328 to Lang et al. and in U.S. Pat. No. 6,122,299 to DeMars et al.
U.S. Pat. No. 6,052,213 to Burt et al. was concerned with fabricating a diffraction grating in a semiconductor wafer, which can in principle replace a bulk optical grating component. A problem with this patent is that the 2D pattern is limited to no more than 10 rows, preferably 1–3 rows and the grating is used simply to disperse the incoming light at grazing incidence to the sample surface.
“Two-dimensional photonic band-gap defect mode laser”, O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O'Brien, P. D. Dapkus, and I. Kim, Science, vol. 284, pp. 1819–1821 (1999), discloses photonic bandgap lasers and optical components employing 2D photonic crystals as a means of suppressing spontaneous emission into unwanted optical modes or creating a microcavity or waveguide. The disclosed device seeks to achieve a large photonic bandgap, usually at as wide a range of angles as possible. High output power can be combined with good beam quality in the tapered-laser design of “1.9-W quasi-CW from a near-diffraction-limited 1.55-μm InGaAsP—InP tapered laser”, S. H. Cho, S. Fox, F. G. Johnson, V. Vusiricala, D. Stone, and M. Dagenais, IEEE Photon. Technol. Lett. (1998). The tapered laser consists of two distinct sections of the optical waveguide. The first section is a single-mode ridge waveguide, and the second is a funnel-shaped gain-guided region. A combination of high-reflectivity (HR) and antireflection (AR) coatings insures that nearly all of the laser light emerges from the tapered section end. However, the output power of tapered lasers is constrained by the maximum aperture that can produce diffraction-limited output for a given injected current density and the design becomes less attractive for materials with substantial carrier-induced refractive-index fluctuations.
Single-mode operation can also be obtained with a narrow-stripe distributed Bragg reflector (DBR) laser, in which the distributed feedback is confined to mirror-like gratings at one or both ends of the optical cavity. While the grating does not extend into the central region, the feedback from the DBR mirrors is wavelength-selective unlike that in Fabry-Perot lasers. DBR lasers face the same problems as DFB emitters with regard to deterioration of the beam quality and side mode suppression as the stripe is broadened. Widely tunable lasers are known, e.g. as disclosed in “Optimization of the carrier-induced effective-index change in InGaAsP Waveguides—Application to tunable Bragg filters”, J. P. Weber, IEEE J. Quantum Electron. (1994). However, the spectral range is limited by the magnitude of the interband and intervalence absorption contributions to the refractive index, since the plasma shift is relatively small at the telecommunications wavelength of 1.55 μm. Sampled-grating DBR lasers, in which the tuning range has reached 72 nm are also known, as disclosed in “Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings”, V. Jayaraman et al., IEEE J. Quantum Electron (1993). A drawback with this type of laser is that very narrow ridge waveguides are required in order to assure lateral coherence of the laser beam.
None of these approaches provide guidance for identifying the optimized parameters (aspect ratio, etch depth, grating feature size etc.) for structures with 2D gratings. They also do not consider including all three relevant diffraction processes for the rectangular lattice, or, apart from Kalluri et al., tilting the grating.